50.数据库数据查询例题

2、查询“生物”课程比“体育”课程成绩高的所有学生的学号。
# 所有生物课程，体育课程的人的学号和成绩分别筛选出来作为临时表，根据学号连接两个临时表
select A.student_id,bio,phi from

(select student_id,num as bio from score left join course on score.course_id = course.cid where course.cname = ‘生物‘) as A 

left join
 
(select student_id,num as phi from score left join course on score.course_id = course.cid where course.cname = ‘体育‘) as B

on A.student_id=B.student_id where bio>phi;


3、查询平均成绩大于60分的同学的学号和平均成绩； 
select student_id,avg(num) from score group by student_id having avg(num)>60;

4、查询所有同学的学号、姓名、选课数、总成绩；
select score.student_id,student.sname,count(score.course_id),sum(score.num) from 
score join student on score.student_id=student.sid group by score.student_id;

5、查询姓“李”的老师的个数；
select tname from teacher where tname like ‘李%‘;


6、查询没学过“李平”老师课的同学的学号、姓名；
# 先查取‘李平老师’教的所有课ID，再获取选过课的学生ID
select * from student where sid not in(
select distinct student_id from score where course_id in
(select cid from course left join teacher on course.teacher_id=teacher.tid where tname=‘李平老师‘));


7、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
select student_id,sname from
(select student_id,course_id from score where course_id = 1 or course_id = 2) as B     
left join student on B.student_id = student.sid group by student_id having count(student_id) > 1;

8、查询学过“叶平”老师所教的所有课的同学的学号、姓名；
# 和6相同

9、查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
# 同7

10、查询有课程成绩小于60分的同学的学号、姓名；
select sid,sname from student where sid in (select distinct student_id from score where num<60);

11、查询没有学全所有课的同学的学号、姓名；
select student_id,sname
        from score left join student on score.student_id = student.sid
        group by student_id HAVING count(course_id) = (select count(1) from course);
 

12、查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；
select student_id,sname,count(course_id)
from score left join student on score.student_id= student.sid
where student_id !=1 and course_id in (select course_id from score where student_id=1) group by student_id;


13、查询至少学过学号为“001”同学所选课程中任意一门课的其他同学学号和姓名；

14、查询和“002”号的同学学习的课程完全相同的其他同学学号和姓名；


15、删除学习“叶平”老师课的score表记录；
delete from score where course_id in(
		select course.cid from course left join teacher on course.teacher_id=teacher.tid where teacher.tname = ‘李平老师‘);

16、向score表中插入一些记录，这些记录要求符合以下条件：①没有上过编号“002”课程的同学学号；②插入“002”号课程的平均成绩； 
insert into score(student_id,course_id,num) select sid,2,(select avg(num) from score where course_id=2)
from student where sid not in
(select student_id from score where course_id=2);

17、按平均成绩从低到高显示所有学生的“生物”、“物理”、体育”三门的课程成绩，按如下形式显示： 学生ID,生物,物理,体育,有效课程数,有效平均分；
select sc.student_id,
        (select num from score left join course on score.course_id = course.cid where course.cname = "生物" and score.student_id=sc.student_id) as sy,
        (select num from score left join course on score.course_id = course.cid where course.cname = "物理" and score.student_id=sc.student_id) as wl,
        (select num from score left join course on score.course_id = course.cid where course.cname = "体育" and score.student_id=sc.student_id) as ty,
        count(sc.course_id),
        avg(sc.num)
    from score as sc
    group by student_id desc;    

18、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
select course_id,max(num),min(num) from score group by course_id;

19、按各科平均成绩从低到高顺序；
# 按照科目分组
select course_id,avg(num) from score group by course_id order by avg(num) asc;	

20、课程平均分从高到低显示（显示任课老师）；
select avg(score.num),teacher.tname from course
left join score on course.cid=score.course_id
left join teacher on course.teacher_id=teacher.tid
group by score.course_id;

21、查询各科成绩前三名的记录:(不考虑成绩并列情况) 
 select score.sid,score.course_id,score.num,T.first_num,T.second_num from score left join
    (
    select
        sid,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 3,1) as second_num
    from
        score as s1
    ) as T
    on score.sid =T.sid
    where score.num <= T.first_num and score.num >= T.second_num;


22、查询每门课程被选修的学生数；
select course_id,count(student_id) from score group by course_id;

23、查询出只选修了一门课程的全部学生的学号和姓名；
select score.student_id,student.sname from score left join student on student.sid=score.student_id group by course_id having count(course_id)=1;

24、查询男生、女生的人数；
select gender,count(sid) from student group by gender; 


25、查询姓“张”的学生名单；
select * from student where sname like ‘张%‘;


26、查询同名同姓学生名单，并统计同名人数；
select sname,count(1) from student group by sname;

27、查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列；
# 按课程分组
select course.cname,avg(num) as avg from score left join course on course.cid=score.sid group by course_id order by avg asc,course_id desc;


28、查询平均成绩大于85的所有学生的学号、姓名和平均成绩；
select score.student_id,student.sname,avg(num) from score left join student on student.class_id=score.course_id group by student_id having avg(num)>85;

29、查询课程名称为“生物”，且分数低于60的学生姓名和分数；(居然还能连续使用join)
select student.sname,score.num from score 
left join course on course.cid=score.course_id 
left join student on score.student_id=student.sid
where score.num<60 and course.cname=‘生物‘;


30、查询课程编号为003且课程成绩在80分以上的学生的学号和姓名；
select score.student_id,student.sname,score.num from score left join student on student.sid=score.student_id where score.course_id=3 and score.num>80; 

31、求选了课程的学生人数(记得去重)
select count(distinct student_id) from score; 

32、查询选修“张磊”老师所授课程的学生中，成绩最高的学生姓名及其成绩；
select student.sname,score.num from student left join score on student.sid=score.student_id where score.course_id in(select course.cid from course left join teacher on course.teacher_id=teacher.tid where teacher.tname=‘张磊老师‘)order by num desc limit 1; 

33、查询各个课程及相应的选修人数；
select course.cname,count(1) from score left join course on score.course_id=course.cid group by course_id;


34、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；
select distinct s1.student_id,s2.course_id,s1.num from score as s1,score as s2 where s1.num = s2.num and s1.course_id != s2.course_id;

35、查询每门课程成绩最好的前两名；
# 以课程分组
# 升序排列order by num desc     
# 筛选limit 0,1

  select score.sid,score.course_id,score.num,T.first_num,T.second_num from score left join
    (
    select
        sid,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 1,1) as second_num
    from
        score as s1
    ) as T
    on score.sid =T.sid
    where score.num <= T.first_num and score.num >= T.second_num;

36、检索至少选修两门课程的学生学号；
select student_id from score group by student_id having count(course_id)>1; 

37、查询全部学生都选修的课程的课程号和课程名；
# 以课程分组

select course_id,count(1) from score group by course_id having(1)=(select count(1) from student);


38、查询没学过“叶平”老师讲授的任一门课程的学生姓名；
select student_id,student.sname from score 
left join student on score.student_id = student.sid
where score.course_id not in(select cid from course left join teacher on course.teacher_id = teacher.tid where tname=‘张磊老师‘) group by student_id;

39、查询两门以上不及格课程的同学的学号及其平均成绩；
select student_id,avg(num) from score group by student_id;

select student_id,count(1) from score where num<60 group by student_id having count(1)>=2;


40、检索“004”课程分数小于60，按分数降序排列的同学学号；
select student_id from score where student_id=4 and num<60 order by num desc;

41、删除“002”同学的“001”课程的成绩；
delete from socre where student_id=2 and course_id=1;